Radio paging systems can be one-way systems or two-way systems. In either paging system, a given service area will usually include a number of fixed site transmitters for sending information to a larger number of portable selective call receivers, such as pagers.
The one-way paging system typically operates in a simulcast mode in which a group of transmitters simultaneously transmit the same FM (frequency-modulated) information to selected pagers within the service area. Thus, a pager within the service area will typically be exposed to signals from more than one transmitter. If one such signal is sufficiently stronger than the others, the strongest signal will be "captured", the other signals will be rejected, and the pager will decode the information contained in the "captured" signal. If a pager in a one-way paging system is in a region where it receives two or more signals of nearly equal strength, it is likely that no "captured" will occur. However, if all transmitted signals contain the same information, the information contained in one received signal does not necessarily corrupt the information contained in another received signal. Consequently, the pager can usually decode the received information, except in a situation where there is a large phase delay between received signals. If such a phase delay exists, and the pager did not "captured" one of the transmitted signals, then the pager may not be able to decode the information that was sent to it.
A similar problem can arise in a two-way paging system in which a number of fixed-site transmitters simulcast frequency-modulated information to a number of pagers within the service area. The transmitted information includes an identification code (sometimes referred to as a color code or transmitter ID) that is unique to the transmitter (or unique to the transmitter site). If a pager "captured" one of the transmitted signals, it will decode the transmitter's identification code and send an acknowledgment signal back to fixed-site receivers. This acknowledgment signal will normally include the transmitter identification code which was decoded by the acknowledging pager. This allows the system to infer that the acknowledging pager is nearest to the transmitter whose identification code was decoded by that pager. That nearest transmitter can then be used to send, to that acknowledging pager, further information (such as messages) that are intended for its user.
On the other hand, if the pager cannot "capture" one of the signals and reject the other signals (because two or more received signals are nearly of equal strength), the pager will be presented with two or more signals that contain different transmitter identification codes. In this situation, the different received identification codes can corrupt each other, resulting in the pager being unable to properly decode any one of the transmitter identification codes. When this happens, the pager's acknowledgment signal will not include a proper transmitter identification code, in which case the system will not be able to determine which transmitter is closest to the pager. Merely re-sending the same information in the same way could give the same undesirable results. This problem could be avoided if all pagers in the system could reliably "capture" the signals that they are intended to receive.